Method of forming a chemical phosphate coating on the surface of steel

Abstract

A method of forming a chemical phosphate coating on the surface of steel with a treatment bath made without directly adding any oxidizing agent such as nitrite ions or hydrogen peroxide, and maintaining its temperature in a range not exceeding 40° C., its pH in the range of 2.5 to 4.5 and its oxidation-reduction potential (ORP in terms of the normal hydrogen electrode potential) in the range of 150 to 550 mV. The bath is formed from an aqueous solution of the two components, i.e. the first component, an acidic solution consisting mainly of oxo acid ions, such as, NO 3 - , phosphoric ions (H 3 PO 4 or H 2 PO 4 - ), and metal ions, such as Zn 2+ , and the second component, an alkaline solution containing hydroxide ions (OH - ). Preferably, the bath has a temperature of 20° C. to 30° C., a pH value of 3.0 to 4.0 and an ORP of 350 to 450 mV.

What is claimed is: 
     
       1. A method for providing a phosphate coating on a succession of steel articles, comprising: (a) constituting a treatment bath by introducing a principal and an auxiliary bath component into a bath container, wherein: (i) said principal bath component is an acidic aqueous solution consisting mainly of H 2  PO 4   - , oxo acid ions and metal ions; and   (ii) said auxiliary bath component is an aqueous alkaline solution containing hydroxide ions,      the relative concentrations of said principal and auxiliary bath components being such that at a temperature in the range of 0° C. to 40° C., said treatment bath has a pH range of 2.5 to 4.5 and an ORP range, in terms of normal hydrogen electrode potential, of 150 to 550 mV, said treatment bath containing at least 2 g of phosphoric ions per liter and no more than 10 ppm free NO 2   -  ;   (b) wetting an exposed surface of each of a succession of steel articles with said treatment bath within said temperature range in order to generate a phosphate coating on said exposed surface of each said steel article,   (c) while conducting said wetting, (i) monitoring the pH of said treatment bath and adding more of said principal component thereto in order to maintain the pH of said treatment bath lowered to within said pH range,   (ii) monitoring the ORP of said treatment bath and adding more of said auxiliary component thereto in order to maintain the ORP of said treatment bath lowered to within said ORP range,   (iii) without adding to said treatment bath either for constituting said treatment bath or for maintaining said pH or ORP thereof any oxidizing agent which reacts within one hour if added to said principal component so as to fail to retain the chemical formula which represented such oxidizing agent prior to its mixture with said principal component.     
     
     
       2. The method of claim 1, wherein: said temperature range is 20° C. to 30° C., said pH range is 3.0 to 4.0 and said ORP range is 350 to 450 mV.   
     
     
       3. The method of claim 2, wherein: said oxo acid ions include at least one of NO 3   -  ions and ClO 3   -   ions; and   said metal ions include those of at least one of zinc, manganese, calcium, magnesium, nickel and a metal which forms a hydrogen phosphate which is stable in an aqueous solution and which is capable of showing a substantial reduction in solubility as a result of dehydrogenation.